Hard disk drives include one or more rigid disks, which are coated with a magnetic recording medium in which data can be stored. Hard disk drives further include read and write heads for interacting with the data in the magnetic recording medium. The write head includes an inductive element for generating a magnetic field that aligns the magnetic moments of domains in the magnetic recording medium to represent bits of data.
Magnetic recording techniques include both longitudinal and perpendicular recording. Perpendicular magnetic recording (“PMR”) is a form of magnetic recording in which the magnetic moments representing bits of data are oriented perpendicularly to the surface of the magnetic recording medium, as opposed to longitudinally along a track thereof. PMR enjoys a number of advantages over longitudinal recording, such as significantly higher areal density recording capability.
Write poles with a trapezoidal cross-sectional shape at the air bearing surface (“ABS”) are used to provide improved writing performance in PMR heads. The manufacture of write poles with this trapezoidal cross-sectional shape presents a number of difficulties, however. One approach to manufacturing such poles involves a reductive process of milling poles from a layer of magnetic material. Due to the complex three-dimensional shapes called for in next-generation hard disk drives, however, this process can be extraordinarily difficult and prone to low yields. Another approach to manufacturing these poles involves an additive process, in which damascene trenches are formed in an insulating substrate layer and filled with a magnetic material. The success of this process relies upon the formation of a properly dimensioned and shaped damascene trench.
One approach to forming a damascene trench involves providing a hard mask with an opening over a region of insulating substrate and removing the portion of the substrate below the opening. For example, as shown in FIG. 1a, a hard mask with two layer 105 and 104 (e.g., of Cr and Ru or NiFe) may be provided over an iso-line of photoresist 107 and a bar layer 106 (e.g., of nitride). The hard mask covers a layer of insulating substrate 102 (e.g., Al2O3) and, optionally, a secondary mask layer 103 (e.g., of Ta). The insulating substrate 102 may itself be provided over a lower substrate layer, such as, for example, NiCr. The iso-line of photoresist 107 is provided over a region of insulating substrate 102 in a region where a damascene trench will be formed. By side-milling the structure illustrated in FIG. 1a, the portion of the hard mask layers 105 and 104 covering photoresist 107 can be removed, and by removing photoresist 107 and bar layer 106 via lift-off, a patterned hard mask with an opening 108 is formed over the substrate 102, as is shown in FIG. 1b. When this structure is subjected to a reactive ion etching (RIE) operation, the pattern opening 108 is transferred to insulating substrate 102, forming a damascene trench 109, as is shown in FIG. 1c. 
This process of side-milling an opening in a hard mask suffers from a number of drawbacks. For example, if NiFe is used as a hard mask material, the subsequent removal of excess NiFe from the structure (which is necessitated by NiFe's magnetic properties) requires wet-etching, which can adversely impact the shape and dimensions of the damascene trench. Ru may make a better material for the hard mask, giving the RIE selectivity between Ru and Al2O3 (i.e., in an RIE operation in the presence of chlorine gas used to form a damascene trench in an Al2O3 layer protected by a Ru hard mask, the Ru will etch at a much slower rate than the Al2O3), but the process of forming the hard mask via side-milling and lift-off may still leave the Ru hard mask layer with fencing (i.e., the upward-thrusting hard mask material left over following side-milling and lift-off, illustrated in FIG. 1b, which can adversely impact the critical dimension and/or side wall angle of the damascene trench. Moreover, the milling process may roughen the upper surface of the hard mask material, which can reduce the effectiveness and/or predictability of a subsequent chemical-mechanical polishing (CMP) step used to remove the mask layer. Finally, the processes of side-milling and lift-off require a different process chamber than does the process of RIE, increasing the complexity of the foregoing hard mask formation method.